Process of recovering alumina from coal and its ashes



Patented Aug. 23, 1932 MT 'ST ES as FFICE r WILLIAM M. GUERTLER, OF BERLIN, GERMANY, ASSIGNOR TO THE ELECTRIC SMELTING 6t ALUMINUM COMPANY, OF CLEVELAND, OHIO, A CORPORATION OE OHIO rnocnss or nroovnnme ALUMrnnrnoM coAL nn r rs ASHES No Drawing.

This invention relates to the recovery of alumina from coal or coal ashes. I

An object of my invention is to secure alumina from coal or coal ashes in a simple and efficient manner and bya process in which the various residues obtained in the extraction of the alumina are valuable and useful commercial products.

A further object of my invention is to secure alumina from coal or coal ashes use of an acidlprocess which due to the valu able byproducts obtained according to my process, enables me to secure alumina at a very low cost from products hitherto not used for this purpose. 7 3

Another object of my invention is particularly to recover alumina by anefficient process from coal ashes, which has heretofore been considered a waste product. v

In modern power plants steam is generated by burning bituminous coal in boilers either by the use of mechanical stokers or by the burning of finely powdered coal. In this last system the coal is ground to a very fine condition and is then burned by means of special burners whereby the coal is propelled into an incandescent fire box t'ogether with air whereupon the carbon of the coal islargely burned out producing a'very hot flame. The ash of the coal remains suspended in the flue gases and by virtue of its position and of the very high temperature of the flame, theash particles become more or less globular-in shape, fused to semi-fused and certain chemical and physical transformations take place which are discussed later.

The ash content of bituminous coal varies from three percent to fifteen percent of the coal weight and since the coal is very finely divided prior to burning it will be appreciated that the ash particles formed will be much finer even than the coal. r

Samples have been observed which show that 99% of the ash passes through a 100 meshscreen and 90% througha 300' mesh scraped to a bin underneath;

Application filed March 21, 1830. Serial No. 437,948.

screen. It is very difficult to remove such finely divided particles from the hot flue gases but such removal has in some instances been required for when the ashes have been discharged through the stacks of the power,

plants they have gradually settled on the surrounding country to the detriment of vegetation, buildings and people in the neighborhood. Power plants such as are located in or near. large cities sometimes separate hundreds of tons of such ash from their stack gases each twenty-four hours; A method for such separation is by means of a Cottrell electrostatic dust precipitator. The disposal of this finely divided ash has been a serious problem and a source of expense.

In the operation of the Cottrell electrostatic dust precipitator for the separation of the ash particles from the hot gas stream, the flue gases from the boilers are drawn by means of a suction fan through this device; which consists essentially of positive and negative poles where, by means of a voltage of 100,000 to 150,000 volts an electrical discharge passes through the moving gas stream and a charge or potential is thereby placed on the ash particles which causes them to'be attracted to one pole and to be deposited thereupon from which they fall or are The temperature of the flame produced in burning powdered coal is upward of 2000" 0., but the temperatureof the flue gases "falls rapidlyin the boilers andfiues until in the dust precipitator the temperature of the gases is only about 500 C. This high initial temperature and comparatively rapid cooling produces certain chemical andphysical transformations and produces certain minerals or crystal forms in the ash particles.

The effect of these chemical andphysical" transformations is to make the alumina content of the ashes more readily soluble 1n suitable mineral acids, as Wlll be hereinafter explained.

the electrostatic discharge in the dust pre M ,cipitator although this is not definitely known. At any rate the ash produced as de-' scribed has certain properties different from the original minerals and different from most naturally occurring minerals. First, the iron content of the original coal forms and separates into a magnetic oxide of iron, largely F6 0 which may be largely separated from the ash by means of a magnetic separator and which then becomes a product of value in the art as a paint pigment or as iron ore of very high metallic iron content. Second, the ash may be treated with mineral acids such as HCl, HNO S6 or HClO or combinations of these to dissolve the base minerals such as alumina, iron oxide, calcium or magnesium oxides, etc., leaving a residue which is essentially iron free silica and which forms an improved raw material for cement and chemical manufacture.

It is well known that the solubility of alumina and basic content of clays, and similar minerals in mineral acids, may be increased by roasting them within a very narrow range suitable for the particular clay in question. Such a range is usually less than 100 C. in order to secure the most eflicient extraction of the basic content of the clay. If, however, these clays are heated to a temperature of approximately 900 0., or above, the basic content is practically insoluble, and in some cases less than 1% can be extracted by subjecting the roasted clay to the same 1nin- 4 5 eral acid treatment as will extract of the basic content if the proper roasting temperature is observed.

I have found that a high extraction of the bases may be readily obtained from the ashes from powdered coal with the use of suitable mineral acids, notwithstanding the fact that such ashes have been heated to temperatures considerably more than 900 C. The reason for this may be that the ashes are partially fused and rapidly cooled or quenched, and thus the solubility of the basic content increased.

I have also found that the same condition obtains in powdered ashes derived from the grinding of clinkers from the burning of coal. With coal ashes derived either from the burning of powdered coal or bituminous coal, which have, in the ordinary furnace practice, attained a temperature in excess of 900 0., and thus become partially fused or sintered, extractions of or more of the basic content have been obtained,

The usual range of composltlons of the above described ashes and of one typical sample is given below:

Per cent Per cent SiO 4:0-5O 46.9 A1 0 25-s5 29. F e O FeO 915 11.7 g0 0.l1.3 .9 CaO 23 2.6 TiO l 1.3 Carbon 58 6.5 Unclassified (D) .8

The composition of the ashes produced is variable due to the fact that different coals used as fuel vary as to their ash content and as to the composition of such ash, and also due to the fact that the conditions under which the coal is burned are variable, the flame temperature fluctuating.

In carrying out my process a slurry of the powdered coal ash and water is formed and this slurry can be pumped and stored in tanks and treated without serious difficulty. One convenient method of removing the magnetic iron is to pass this slurry through a magnetic separator of any suitable design and separate the magnetic oxide of iron which can readily be disposed of as a paint pigment or as iron ore of high iron content.

I then treat the material from which the iron content has been largely removed with a suitable quantity of mineral acid, either I-ICl, HNO H st) or HClO or combinations of these in such amount as to dissolve the remaining bases from the ash. Heat generated by the dissolving reaction is usually suflicient to maintain the solution hot, but I can heat the solution if desired.

I have found that one good method for carrying out this process is by the use of hydrochloric acid in an aqueous solution in a strength of from 5 to 20%, depending upon the physical properties and the analysis of the particular coal ashes. It is to be understood, however, that two or more of the above acids may be used simultaneously, 1f deslred.

By addlng more or less acld, and varying the time and temperature of treatment, I can i tially all the basic content of the material. in which case the extraction is carried to substantial completion, leaving a residue composed mainly of silica hydrate. In case it is desired to use this residue for the manufacture of sodium silicates, the silica hydrate can be digested with caustic soda or heated to fusion with soda ash, as is well known.

After suitable treatment of the ashes with the acid solution, the dissolved aluminum content may be readily separated; from the residue by filtration or any other desired means. After removal by well known means of the other basic content of the solution such as calcium, magnesium and iron oxide, if the latter has not been previously removed, the alumina content can be precipitated as hydroxide for conversion to alumina by well known processes, such as by ignition of the aluminum hydroxide and this can be accomplished with the regeneration of the acid used to dissolve the alumina and the acid that has been chemically combined therewith.

Furthermore, it is to be understood that the particular compounds disclosed, andthe procedure set forth, are presented for purposes of explanation and illustration and that various equivalents can be used and modifications of said procedure can be made without departing from my invention as defined in the appended claims.

What I claim is:

1. The process of producing alumina from coal in which at least some iron is present, which consists in burning the coal above the temperature of 900 0., forming a water slurrythereof, removing the magnetic iron from the water slurry, subjecting the ashes to the action of an aqueous mineral acid for a sufficient t me to secure the desired extraction of alumina, separating the solution from' the residue, and precipitating alumina from the solution.

2. The process of obtaining alumina from coal ashes in which at least some iron is pres ent which have been at least partially fused, which consists in subjecting the finely divided ashes to the action of a magnetic separator to remove the magnetic oxides of iron, subjecting the ashes so treated to the action of an aqueous mineral acid for a suflicien't time to secure the desired extraction of alumina, separating the solution from the residue and precipitating alumina from the solution.

3. The process of producing alumina from the partially fused ashes obtained from the burnin of powdered coal in which at least some iron is present, which comprises extracting the magnetic oxides of ironfrom such ashes and dissolving the alumina content in hydrochloric acid, separating the acid solution from the residue, precipitating the alus mina from the solution and recovering the acid.

4:. The process of producing alumina from tracting the magnetic oxides of iron fromthe ashes, subjecting the ashes to a mineral acid solution to extract substantially the complete basic content of the ashes, treating the solution to remove the metallic content of the I solution other than the aluminum salt, forming aluminum hydrate and separating it from the solution.

5. The process of producing alumina from the partially fused ashes obtained from the burning of powdered coal in which at least some iron is present, which consists in forming a water slurry thereof, passing the water slurry through a magnetic field to remove the magnetic oxides'of iron, subjecting the ashes to a mineral acid solution to extract substantially the complete basic content of the ashes, treating the solution to remove the metallic content of the solution other than the aluminum salt, forming aluminum hydrate and separating it from the solution.

6. The process of recovering the aluminous content from finely powdered coa in which at least some iron is present, which consists in burning the coal at a temperature of at least 900 C. to produce partially fused ashes, mixing the ashes with water to form a slurry, and extracting the magnetic iron content of the ashes from the slurry, then adding a mineral acid to the slurry to dissolve the aluminous content of the ashes, and separating the aluminous solution from the silicious residue.

7. The process of recovering the'aluminous content from finely powdered coal in which at least some iron is present, which consists in burning the coal at a temperature of over 900 (1, thereby forming substantially fused ashes, rapidly cooling the ashes, mixing the ashes with water to form a slurry and extracting the magnetic iron content of the ashes from the slurry, then adding hydrochloric acid to the slurry to dissolve the aluminous content and thereby form an aluminous solution, and separating the aluminous solution from the residue.

8. The process of recovering the aluminum content from finely powdered coal in which at least some iron is present, which comprises burning the coal at a temperature substantially above 900. 0.,thereby forming ashes in a sintered or fused state, cooling the ashes rapidly, thereby rendering the aluminum content of the ashes highly soluble in mineral acids and forming magnetic oxide of iron, removing the magnetic oxide of iron as a byproduct, adding a mineral acid to the fused ashes to dissolve the aluminum content, and separating the aluminum solution so formed from the residue.

In testimony whereof I affix my signature.

lWILLIAM M. GUERTLER. 

